How do you create a level 21 / 23 quality herald of ice skill gem?

its chance when you use the lapidary lens on a 20/20 gem (t3 gem room in alvas temple)

or you could just corrupt your 20 lvl 20% quality gem with vaal orb.

Don't pay attention to a guy above me. Just drop vaal orb on a fully leveled gem.

P.S. Most likely it will either remain the same or become 21/20 or 19/20 or 20/19...
21/23 is the best and probably rarest possible outcome that is why they are so highly valued

Corrupting a gem, Outcomes:

Nothing happens, but the gem gets a “corrupted” tag
Gain 1 gem level
Lose 1 gem level
Gain up to 10% quality (caps at 23%)
Lose up to 10% quality
Turn the gem into a Vaal version. For example, Vaal Spark.

Using a Lapidary Lens applies 2 of the 6 at once.

It doesn’t work that way.

This is wrong, its funny that you are shunning someone else to give shit information

how does it work then? i would like to upgrade some of my gems...

So, you are saying corrupting a skill gem with vaal does not have a chance to turn 21/23 gem?

That's exactly what I am saying.



Only way to get 21/23 is the Alva room.

If not from incubators or other really rare occurrences (drops)

Exactly, the only way to double corrupt a gem is through alva temple. Or you can get lucky and have on dropped. In that case it can even be a vaal 21 23 (triple corrupted) vaal orb can influence 1 stat at max!